I want to select surfaces whose backfaces are visible from the current perspective viewpoint.

This would be useful for flipping the direction of surfaces that are difficult or tedious to select manually.

3ds Max has a similar function in Editable Poly sub-object Face mode, where you can select faces that are oriented within a specified angle to the current perspective. There should be a way to do this in Rhino, but not necessary to include the angle tolerance.. only visible back faces of surfaces.